R. M. Arthur, J. W. Trobaugh, W. L. Straube, E. G. Moros and S. Sangkatumvong, "Temperature Dependence of Ultrasonic Backscattered Energy in Images Compensated for Tissue Motion", Proceedings of the 2003 International IEEE Ultrasonics Symposium", Honolulu, Hawaii, October 5-8, 2003.

Abstract

Noninvasive temperature imaging would enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. Previously, we predicted monotonic changes in ultrasonic backscattered energy (CBE) for certain sub wavelength scatterers. We measured CBE values similar to our predictions in bovine liver, turkey breast, and pork rib. Those 1D measurements were corrected manually for changes in the axial position of scatterers with temperature. To investigate the effects of apparent motion on CBE in 2D, we imaged 1-cm thick samples of beef liver during heating in a water bath from 37 to 50^oC. Images were formed by a Terason 2000 imager with a 7 MHz linear probe focused at 4.5 cm, the center of the tissue specimen. Employing RF signals from the Terason 2000 (courtesy Teratech Corp.) permitted the use of cross-correlation as a similarity measure for automatic tracking of feature displacement as a function of temperature. Tissue motion across the specimen was non-uniform with typical total displacements of 0.5 to 1mm in both axial and lateral directions. Tissue motion in image regions of about 15 mm² was tracked from 39 to 50^oC in 0.5^oC steps. Motion compensated image regions were demodulated with the Hilbert transform and smoothed with a 3x3 running average filter. Values were squared to form the backscattered energy at each pixel. Our measure of CBE compared standard deviations of the BE images. CBE increased monotonically to about 4 dB at 50^oC from its value at 39^oC. The CBE curve was matched by a second degree polynomial with ρ > 0.99. Relatively noise-free CBE curves from tissue volumes of less than 1 cm³ supports the use of CBE for temperature estimation. Motion in 3D will affect CBE values, but that motion can tracked as it was in this study. Because beam width in elevation is larger than the lateral width, effects of motion in elevation on CBE may be less. Thus, we expect CBE to support temperature estimation in 3D.

Acknowledgement: This work was supported in part by NIH grant R21-CA90531 from the National Cancer Institute and the Wilkinson Trust at Washington University in St. Louis.